Fibers having a small fiber diameter can impart various excellent properties, such as a separating property, a liquid-holding capacity, a wiping property, an opacifying property, an insulating property, or flexibility, to a nonwoven fabric, and therefore, it is preferable that a nonwoven fabric is composed of fibers having a small diameter. As a process for manufacturing such fibers having a small fiber diameter, electrospinning is known. In this process, a spinning solution is extruded from a nozzle, and at the same time, an electrical field is applied to the extruded spinning solution to thereby draw the spinning solution, and fibers having a small fiber diameter are spun. When a spinning solution containing an organic polymer is used in electrospinning, fibers made of the organic polymer can be produced.
Such an organic fiber having a small fiber diameter has a low mechanical strength due to its small fiber diameter, and therefore, a nonwoven fabric composed of the organic fibers is hard to handle. The present inventors attempted to improve the mechanical strength of the organic fiber by adding an inorganic component to the organic fiber, together with the addition of a function imparted by the inorganic component.
As a technique to add a function, but not to enhance a mechanical strength, for example, patent literature 1 discloses “a nanofiber having a hetero-phase structure obtained by electrospinning, wherein the hetero-phase structure contains a first phase which extends along with the central axis of a fiber, and a second phase which is arranged at the outside of the first phase, with respect to a cross section vertical to the longitudinal direction of the first phase, and covers the first phase; the first phase is a region made of a first material containing an inorganic material; and the second phase is made of a second material different from the first material”. Patent literature 1 also discloses that the nanofibers can be obtained by electrospinning, i.e., by supplying two different types of fluids to a spinneret, and applying a DC voltage between the spinneret and a target. The present inventors carried out the process under conditions that the content of the inorganic material accounted for 10% or less (which is considered to be preferable to enhancing a fiber strength) with respect to the whole mass of the nanofibers, but nanofibers having the two-layered hetero-phase structure could not be obtained, and therefore, the mechanical strength of a nonwoven fabric could not be enhanced.
Neither to enhance a mechanical strength nor to add a function, patent literature 2 discloses “a method for obtaining a fiber structure by extruding a solution prepared by dissolving a fiber-forming solute such as an organic macromolecule or a ceramic precursor compound in a solvent to an electrostatic field generated between electrodes, spinning the solution toward the electrodes, and accumulating formed fibrous substances on a support for capture”. Patent literature 2 also discloses in Examples that “ion-exchanged water was added to a solution previously prepared by adding acetic acid to titanium n-butoxide to generate a gel, polyethylene glycol was added to the gel to prepare a spinning solution, and the spinning solution was used to carry out spinning”. According to the Examples, fibers comprising an organic component and an inorganic component could be produced, but the inorganic component existed in the fiber as inorganic particles, because the organic component was mixed with the generated gel, and therefore, the mechanical strength of the organic fiber could not be enhanced, and as a result, the mechanical strength of a nonwoven fabric could not be enhanced.